1. Field of the Invention
The invention relates to a hydraulic power steering system that assists an operation for steering a vehicle using a force produced by a hydraulic cylinder.
2. Description of Related Art
A hydraulic power steering system that assists an operation for steering a vehicle using a hydraulic cylinder includes a selector valve that is used to control the manner in which a hydraulic fluid is supplied to the hydraulic cylinder and drained from the hydraulic cylinder, in accordance with the amount of steering torque transmitted from a steering shaft to a steered shaft via a rack and pinion mechanism (for example, refer to Japanese Patent Application Publication No. 2006-123580).
How the manner in which the hydraulic fluid is supplied to and drained from the hydraulic cylinder should be controlled by the selector valve is determined based on the relative phase difference between a shaft fixed to the steering shaft and a rotary valve connected to the shaft via a torsion bar, that is, the torsion amount of the torsion bar.
With the conventional hydraulic power steering system structured as described above, the driver's steering operation is appropriately assisted. However, the assist force, that is, the manner in which the hydraulic fluid is supplied to and drained from the hydraulic cylinder is determined based simply on the torsional torque of the steering shaft. Therefore the assist force cannot be flexibly changed to meet various requirements based on other vehicle operation states. Accordingly, such a conventional hydraulic power steering system has room for improvement in changing the assist force that is produced by the hydraulic cylinder flexibly enough to meet various requirements.